1. Field of the Invention
The present invention generally relates to a cellular wireless communication system. More particularly, the present invention relates to an apparatus and method for allocating resources to control information in a cellular wireless communication system.
2. Description of the Related Art
Mobile communication systems were developed to enable users to conduct communications with mobility. The rapid development of technologies has driven the development of the mobile communication systems to provide high-speed data service as well as voice service. Mobile communication systems have been evolving rapidly in order to meet demand for high-speed data service. One such example is Enhanced Universal Terrestrial Radio Access (EUTRA), the future-generation mobile communication standard of the 3rd Generation Partnership Project (3GPP).
Various multiple access schemes are available to mobile communication systems, including Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Frequency Division Multiple Access (FDMA). Among them, CDMA is popular. However, CDMA has limitations in transmitting a large volume of data at a high rate due to a limited number of orthogonal codes. At present, Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier-FDMA (SC-FDMA), which are special cases of FDMA, have been adopted as the respective DownLink (DL) and UpLink (UL) standard technologies of EUTRA.
In the EUTRA system, UL control information includes ACKnowledgment/Negative ACKnowledgment (ACK/NACK) feedback information indicating whether DL data has been received successfully and Channel Quality Indication (CQI) information representing a DL channel state.
The ACK/NACK information is typically 1 bit and is repeatedly transmitted to improve reception performance and expand cell coverage. In general, the CQI information occupies a plurality of bits to indicate the channel state and is channel-encoded prior to transmission to improve reception performance and expand cell coverage. The channel encoding is block coding, convolutional coding, or the like.
The reception reliability requirement of control information depends on the type of the control information. An ACK/NACK requires a Bit Error Rate (BER) of about 10−2 to 10−4, lower than the BER requirement of a CQI, ranging from 10−2 to 10−1.
In the EUTRA system, when a User Equipment (UE) transmits only a UL control information channel without data, a particular frequency band is allocated for control information transmission. A physical channel dedicated to transmission of control information only is defined as a Physical Uplink Control Channel (PUCCH) which is mapped to the allocated frequency band.
With reference to FIG. 1, a PUCCH transmission structure will now be described.
FIG. 1 illustrates a PUCCH transmission structure for carrying UL control information in the 3GPP EUTRA system.
Referring to FIG. 1, the horizontal axis represents time and the vertical axis represents frequency. One subframe 102 is shown in the time domain and a system transmission bandwidth 110 is shown in the frequency domain. A basic UL transmission unit, the subframe 102 is 1 ms, divided into two 0.5-ms slots 104 and 106. Each of the slots 104 and 106 is composed of a plurality of SC-FDMA symbols 111 to 124 and 131 to 137, or 118 to 124 and 138 to 144. In the illustrated case of FIG. 1, one slot has seven SC-FDMA symbols.
A minimum frequency unit is a subcarrier and a basic resource allocation unit is a Resource Block (RB) 108 or 109. The RBs 108 and 109 each are defined by a plurality of subcarriers and a plurality of SC-FDMA symbols. Herein, 12 subcarriers and 14 SC-FDMA symbols occupying two slots form one RB, by way of example. On the DL to which OFDM is applied, one RB is also composed of 12 subcarriers and 14 OFDM symbols.
A frequency band to which the PUCCH is mapped is the RB 108 or 109 at either end of the system transmission bandwidth 110. Under circumstances, a Node B can allocate a plurality of RBs for PUCCH transmission in order to allow a plurality of users to transmit control information. To increase frequency diversity during one subframe, frequency hopping may apply to the PUCCH and that frequency hopping is done on a slot basis. Reference numerals 150 and 160 denote frequency hopping, which will be described in more detail below.
First control information (Control #1) is transmitted in the RB 108 in the first slot 104 and in the RB 109 in the second slot 106 by frequency hopping. Meanwhile, second control information (Control #2) is transmitted in the RB 109 in the first slot 104 and in the RB 108 in the second slot 106 by frequency hopping.
In the illustrated case of FIG. 1, in the subframe 102, Control #1 is carried in the SC-FDMA symbols 111, 113, 114, 115, 117, 138, 140, 141, 142 and 144 and Control #2 is carried in the SC-FDMA symbols 131, 133, 134, 135, 17, 118, 120, 121, 122 and 124. A Reference Signal (RS) is transmitted in pilot SC-FDMA symbols 112, 116, 139, 143, 132, 136, 119 and 123. The pilot signal is a predetermined sequence with which a receiver performs channel estimation for coherent demodulation. The number of SC-FDMA symbols carrying control information, the number of RS SC-FDMA symbols, and the positions of the SC-FDMA symbols illustrated in FIG. 1 may vary depending on the type of control information to be transmitted or depending on system implementation.
UL control information such as ACK/NACK information, CQI information, and Multiple Input Multiple Output (MIMO) feedback information from different users can be multiplexed in Code Division Multiplex (CDM). CDM is robust against interference, compared to Frequency Division Multiplex (FDM).
A Zadoff-Chu (ZC) sequence is under discussion for CDM-multiplexing of control information. Because the ZC sequence has a constant envelop in time and frequency, it has a good Peak-to-Average Power Ratio (PAPR) characteristic and exhibits excellent channel estimation performance in the frequency domain. Also, the ZC sequence is characterized by a circular auto-correlation of 0 with respect to non-zero shift. Therefore, UEs that transmit their control information using the same ZC sequence can differentiate the control information by use of different cyclic shift values of the ZC sequence.
In a real radio channel environment, different cyclic shift values are allocated to different users to multiplex control information, thereby maintaining orthogonality among the users. Hence, the number of multiple access users is determined according to the length of a ZC sequence and cyclic shift values. The ZC sequence is also applied to RS SC-FDMA symbols and enables RSs from different UEs to be identified by use of cyclic shift values of the ZC sequence.
In general, the length of a ZC sequence used for the PUCCH is assumed to be 12 samples, which is equal to the number of subcarriers forming one RB. In this case, there are up to 12 different cyclic shift values for the ZC sequence and up to 12 PUCCHs can be multiplexed in one RB by allocating the different cyclic shift values to the PUCCHs. A Typical Urban (TU) model being a radio channel model considered for the EUTRA system uses cyclic shift values of at least two-sample intervals. This implies that the number of cyclic shift values is limited to 6 or less for one RB. As a consequence, orthogonality is maintained without radical loss among PUCCHs mapped to the cyclic shift values in a one-to-one correspondence.
FIG. 2 illustrates an example of multiplexing CQIs from users by use of different cyclic shift values of a ZC sequence in the same RB, when the CQIs are transmitted on PUCCHs having the configuration of FIG. 1.
Referring to FIG. 2, a vertical axis 200 represents cyclic shift values of the ZC sequence. Under the TU model, up to six channels can be multiplexed in one RB without rapid loss in orthogonality. Hence, six CQIs 202, 204, 206, 208, 210 and 212 (CQI #1 to CQI #6) are multiplexed. In the illustrated case of FIG. 2, the CQIs are transmitted using the same ZC sequence in the same RB, while cyclic shift value ‘0’ (denoted by reference numeral 214) applies to CQI #1 from UE #1, cyclic shift value ‘2’ (denoted by reference numeral 218) applies to CQI #2 from UE #2, cyclic shift value ‘4’ (denoted by reference numeral 222) applies to CQI #3 from UE #3, cyclic shift value ‘6’ (denoted by reference numeral 226) applies to CQI #4 from UE #4, cyclic shift value ‘8’ (denoted by reference numeral 230) applies to CQI #5 from UE #5, and cyclic shift value ‘10’ (denoted by reference numeral 234) applies to CQI #6 from UE #6.
With reference to FIG. 1, mapping between a control information signal and a ZC sequence in the ZC sequence-based CDM transmission scheme of control information will now be described.
Let a ZC sequence of length N for UE i be denoted by g(n+Δi)mod N where n is 0, . . . , N−1, Δi denotes a cyclic shift value for UE i, and i is the index of the UE. Also, let a control information signal to be transmitted from UE i be denoted by mi,k where k is 1, . . . , Nsym. If Nsym is the number of SC-FDMA symbols used for transmission of control information in a subframe, a signal ci, k, n mapped to each SC-FDMA symbol, i.e. an nth sample of a kth SC-FDMA symbol from UE i is given asci,k,n=g(n+Δi)mod N·mi,k  (1)
where k is 1, . . . , Nsym, n is 0, . . . , N−1, and Δi denotes the cyclic shift value of UE i.
In FIG. 1, the number of SC-FDMA symbols used for transmitting control information in one subframe, Nsym is 10, excluding four SC-FDMA symbols for RS transmission. The ZC sequence length N is 12, equal to the number of subcarriers forming one RB. For a single UE, a cyclically shifted ZC sequence is applied to each SC-FDMA symbol and a control information signal to be transmitted is configured by multiplying modulation symbols by the time-domain cyclically shifted ZC sequence, one modulation symbol per SC-FDMA symbol allocated for control information transmission. Therefore, up to Nsym modulation symbols of control information can be transmitted in one subframe. That is, up to 10 control information modulation symbols can be transmitted in the one subframe illustrated in FIG. 1.
The multiplexing capacity of PUCCHs that deliver control information can be increased by adding time-domain orthogonal covers to the above ZC sequence-based CDM transmission scheme of control information. A major example of the orthogonal covers is Walsh sequences. For orthogonal covers of length M, there are M sequences that satisfy orthogonality between them. Specifically, time-domain orthogonal covers apply to SC-FDMA symbols to which 1-bit control information like an ACK/NACK is mapped, thus increasing the multiplexing capacity. In the EUTRA system, use of three SC-FDMA symbols per slot is considered for RS transmission on a PUCCH that delivers an ACK/NACK in order to improve the performance of channel estimation. Therefore, when one slot has seven SC-FDMA symbols, as illustrated in FIG. 1, four SC-FDMA symbols are available for ACK/NACK transmission. The use of the time-domain orthogonal covers is limited to one slot or less, to thereby minimize the loss of orthogonality caused by changes in a radio channel. An orthogonal cover of length 4 is applied to the four SC-FDMA symbols for ACK/NACK transmission, while an orthogonal cover of length 3 is applied to the three SC-FDMA symbols for RS transmission. Users that transmit ACKs/NACKs and RSs are basically identified by their cyclic shift values of a ZC sequence and further identified by their orthogonal covers. Since RSs mapped to ACKs/NACKs in a one-to-one correspondence are required for coherent ACK/NACK reception, the multiplexing capacity of ACK/NACK signals is limited by the RSs. For instance, if up to six cyclic shift values are available in one RB under the TU channel model, a different time-domain orthogonal cover of length 3 can be applied to each cyclic shift value of a ZC sequence used for an RS. As a result, RSs from up to 18 different users can be multiplexed. Considering that ACKs/NACKs correspond to RSs one to one, up to 18 ACKs/NACKs can be multiplexed in one RB. In this case, four orthogonal covers of length 4 are available for ACKs/NACKs and three of them are used. The orthogonal covers applied to the ACKs/NACKs are known to both the Node B and the UE by a preliminary agreement or signaling. The use of time-domain orthogonal covers can increase the multiplexing capacity by three times, compared to non-use of time-domain orthogonal covers.
FIG. 3 illustrates an example of multiplexing ACKs/NACKs from users in the same RB by use of different cyclic shift values of a ZC sequence and additional time-domain orthogonal covers in the above-described PUCCH structure for ACK/NACK transmission.
Referring to FIG. 3, a vertical axis 300 represents cyclic shift values of the ZC sequence and a horizontal axis 302 represents time-domain orthogonal covers. In the TU model, up to six channels can be multiplexed in one RB without rapid loss in orthogonality and three orthogonal covers 364, 366 and 368 of length 4 are additionally used. Hence, up to 18 (6×3) ACKs/NACK channels 304, 306, 308, 310, 312, 314, 316, 318, 320, 322, 324, 326, 328, 330, 332, 334, 336 and 338 (ACK/NACK #1 to ACK/NACK #18) can be multiplexed. In the illustrated case of FIG. 3, the ACKs/NACKs are transmitted using the same ZC sequence in the same RB. For the ACK/NACK transmission, cyclic shift value ‘0’ (denoted by reference numeral 340) and orthogonal cover ‘0’ (denoted by reference numeral 364) apply to ACK/NACK #1 from UE #1, cyclic shift value ‘0’ (denoted by reference numeral 340) and orthogonal cover ‘1’ (denoted by reference numeral 366) apply to ACK/NACK #2 from UE #2, and the cyclic shift value ‘0’ (denoted by reference numeral 340) and orthogonal cover ‘2’ (denoted by reference numeral 368) apply to ACK/NACK #3 from UE #3. In this manner, cyclic shift value ‘10’ (denoted by reference numeral 360) and orthogonal cover ‘0’ (denoted by reference numeral 364) apply to ACK/NACK #16 from UE #16, cyclic shift value ‘10’ (denoted by reference numeral 360) and orthogonal cover ‘1’ (denoted by reference numeral 366) apply to ACK/NACK #17 from UE #17, and cyclic shift value ‘10’ (denoted by reference numeral 360) and orthogonal cover ‘2’ (denoted by reference numeral 368) apply to ACK/NACK #18 from UE #18. The orthogonal covers 364, 366 and 368 are orthogonal codes of length 4 that are mutually orthogonal.
The transmission signal format of ACK/NACK channels illustrated in FIG. 3 is detailed in FIG. 4.
FIG. 4 illustrates a transmission format for transmitting ACK/NACK #5 and ACK/NACK #16 in one slot. Referring to FIG. 4, Wi=[Wi,0 Wi,1 Wi,2 Wi,3] where i=0, . . . , 3 can be a Walsh code of length 4 generated from a Walsh-Hadamard matrix given as
                              [                                                                      W                  0                                                                                                      W                  1                                                                                                      W                  2                                                                                                      W                  3                                                              ]                =                  [                                                                      +                  1                                                                              +                  1                                                                              +                  1                                                                              +                  1                                                                                                      +                  1                                                                              -                  1                                                                              +                  1                                                                              -                  1                                                                                                      +                  1                                                                              +                  1                                                                              -                  1                                                                              -                  1                                                                                                      +                  1                                                                              -                  1                                                                              -                  1                                                                              +                  1                                                              ]                                    (        2        )            
Di=[Di,0 Di,1 Di,2] where i=0, . . . , 2 can be a Fourier sequence of length 3 expressed as
                              [                                                                      D                  0                                                                                                      D                  1                                                                                                      D                  2                                                              ]                =                  [                                                                      +                  1                                                                              +                  1                                                                              +                  1                                                                                                      +                  1                                                                              ⅇ                                      j                    ⁢                                          π                      3                                                                                                                    ⅇ                                      j                    ⁢                                          π                      3                                                                                                                                            +                  1                                                                              ⅇ                                      j                    ⁢                                          π                      3                                                                                                                    ⅇ                                      j                    ⁢                                          π                      3                                                                                                    ]                                    (        3        )            
For example, ACK/NACK symbol b of ACK/NACK channel #5 is multiplied by a sequence 405 [s3, s4, . . . , s12, s1, s2] resulting from cyclically shifting a ZC sequence of length 12 [s1, s2, . . . , s12] by two samples and repeated in SC-FDMA symbols 401 to 404. Then the multiplied sequences are again multiplied by the Walsh sequence chips W1,0 W1,1 W1,2 W1,3 of the orthogonal cover ‘1’ in the SC-FDMA symbols 401 to 404. Meanwhile, ACK/NACK symbol b of ACK/NACK channel #16 is multiplied by a sequence 415 [s11, s12, s1 . . . , s9, s10] resulting from cyclically shifting the ZC sequence of length 12 [s1, s2, . . . , s12] by ten samples and repeated in SC-FDMA symbols 411 to 414. Then the multiplied sequences are again multiplied by the Walsh sequence chips W0,0 W0,1 W0,2 W0,3 of orthogonal cover ‘0’ in the SC-FDMA symbols 411 to 414.
Although orthogonality is well preserved among orthogonal cover codes if a channel experiences weak fading, the orthogonality may be lost when a UE moves fast and thus the level of a signal received in one slot fluctuates greatly between SC-FDMA symbols due to time selective fading. Then interference occurs between ACK/NACK channels to which the same cyclic shift value is applied. For example, if UE #1 that transmits ACK/NACK #1 moves fast in FIG. 3, the signal of ACK/NACK #1 interferes with ACKs/NACKs #2 and #3 from other UEs, thereby degrading the reception performance of ACKs/NACKs #2 and #3.